Most imaging systems that impart an image on a media require that the position of the media be known to some degree of accuracy so that the image is correctly positioned on the media. For example when imaging plates for the lithographic printing industry the alignment accuracy requirement is quite precise. In process colour printing, where Cyan, Magenta, Yellow and Black separations must be accurately aligned, the alignment is particularly important. It is common for a Computer-to-Plate (CTP) device to have an accuracy specification of better than ±5 μm between any two plates produced by the same device. In printing, the alignment of an image to a media is commonly referred to as “registration”.
Registration of an image may be achieved by aligning to fixed stops or by optical means or a combination of the two methods. Laser based imaging systems may make use of the write laser beam, or an auxiliary laser beam, to scan over the edge of a media while monitoring the intensity of the reflected light. The edge is found where there is some transition in the reflected light level due to a difference in reflectivity between the media surface and the mounting surface. Such systems are well known in the art and are exemplified by the Trendsetter® 400 Quantum and similar devices sold by Creo Inc of Burnaby, British Columbia, Canada. In these systems the imaging media, which may be an aluminium plate, is secured to the surface of a cylindrical imaging drum. The registration in the circumferential direction is provided by a pair of registration stops disposed in the drum surface. The registration in the direction parallel to the drum axis is determined by locating the edge of the media using a low power red laser beam directed towards the imaging drum surface at an acute angle thereto. A detector, disposed to receive at least the specular portion of the reflection, is continuously monitored while the laser beam is scanned over the edge of the media. The edge location is determined on detection of a transition in the reflected signal between the media and drum surface. The drum surface is rendered at least partially absorptive at the laser wavelength in the area that the edge detection occurs to ensure sufficient contrast so that the transition is well defined.
In U.S. Pat. No. 4,518,862 to Dorn, a system for detecting the position of a sheet on a support is described. The system employs a pair of CCD line sensors that straddle the edge of the sheet. The image of the edge may be analysed using standard image processing routines to determine both angle and position. The disclosed solution is only practical in a system where the edge of the sheet is always in the same general locale. In most CTP systems a wide variety of different sized media are used and the provision of sufficient CCD line sensors to accommodate all cases would be prohibitive.
In U.S. Pat. No. 5,280,171 to Halter, a two-detector system for locating the edge of an object is disclosed. A laser beam is directed towards the vicinity of the edge at an acute angle. A first detector is located proximate the laser source. The second detector is positioned approximately over the light spot produced on the working surface. The two signals from the detectors are combined and used to determine the edge location.
In European Patent Application EP 1 081 458 A2 to Elior et al, a method of detecting the rising edge between two smooth surfaces involves focusing a beam of light on the surface closest to the source. A detector is placed so that when the beam impinges on this closest surface, very little light reaches the detector. If the beam is moved to impinge on the second surface, then the out of focus beam is reflected back into the detector thus providing a transition indicative of the rising edge between the smooth surfaces.